DEPARTMENT OF ELECTRICAL POWER ENGINEERING

COLLEGE OF ENGINEERING

POWER ELECTRONICS

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Topics Covered - Pulse Width Modulated Inverters - Performance Parameters

TOPICS COVERED:

DC TO DC CONVERTER

•  Introduction

Buck (Step Down) Converter

•  Assumptions Made for Analysis of the Buck Converter

•  *Circuit Operation Switching Modes

•  Output Voltage Average

•  The Inductor Current Ripple

•  The Load and the Capacitor Current

•  Continuous Conduction Condition

•  The Capacitor Voltage Ripple

Boost (Step Up) Converter

•  Assumptions Made for Analysis of the Boost Converter

•  *Circuit Operation Switching Modes

•  Output Voltage Average

•  The Inductor Current Ripple

•  The Inductor Current

•  Continuous Conduction Condition

•  The Diode and Capacitor Currents

•  The Capacitor Voltage Ripple

Designing DC-DC Converters

•  Converter Design Consideration

PULSE WIDTH MODULATED INVERTERS

•  Introduction

•  Inverter Types

•  Basic Inverter

•  Performance Parameters

•  Fourier Series

•  Total Harmonic Distortion

Single Phase Inverter

•  Single Phase Half Bridge Inverter

•  Single Phase Full Bridge Inverter

•  Square-wave Inverter

•  Square-wave Inverter with RL Load

•  Pulse-Width Modulation (PWM) Control Strategy

•  Amplitude & Harmonics Control

•  Sinusoidal Pulse Width Modulation (SPWM)

•  PWM Definitions & Considerations

Bipolar Switching (Single Phase)

•  PWM Outputs

•  PWM Harmonics

Unipolar Switching (Single Phase)

•  PWM Outputs

•  PWM Harmonics

Three Phase Inverter

•  Introduction

•  Switch State for Six Step Inverter

•  Six Step Inverter (continue)

•  Three Phase SPWM

 

Performance Parameters

Harmonics: a steady state distortion of the fundamental frequency (50Hz).

  • Harmonic distortion of current occurs when sinusoidal voltage is applied to a non-linear load (ex. electronic ballast, PLC, adjustable-speed drive, arc furnace, any ac/dc converter).
  • The result is a distortion of the fundamental current waveform.
  • This distortion occurs in integer multiples of the fundamental frequency (50Hz).
  • Hence, the 2nd Harmonic has a frequency = 2 x 50 = 100Hz, the 3rd Harmonic = 150Hz and so on.
  • On the other hand, voltage distortion is generated indirectly as a result of harmonic currents flowing through a distribution system.
  • Refer to the figures below for further understanding.

Harmonics Filtering

  • Output of the inverter is ˇ°chopped AC voltage with zero DC componentˇ±. It contain harmonics.
  • An LC section low-pass filter is normally fitted at the inverter output to reduce the high frequency harmonics.

Output Voltage Harmonics/ Distortion

  • Harmonics cause distortion on the output voltage.
  • Lower order harmonics (3 rd , 5 th etc) are very difficult to filter due to the filter size and high filter order.
  • They can cause serious voltage distortion.

Why need to consider harmonics?

  1. Sinusoidal waveform quality must match TNB supply.
  2. ˇ°Power Qualityˇ± issue.
  3. Harmonics may cause degradation of equipment. Equipment need to be ˇ°de-ratedˇ±.

 

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